The present invention is concerned with a novel composition which is useful as an additive for oil and gas well drilling fluids, notably, aqueous clay dispersions, or muds, for the purpose of improving fluid rheology and fluid loss control.
The use of drilling fluid such as clay-based dispersions or muds, to coat the walls of well holes, is an old and well-established procedure. The purpose of the fluid is to stabilize the walls of the hole by forming a relatively thin but strongly adherent coating or "cake" on the walls. The coating must be able to withstand relatively high temperatures and it should be resistant to the passage of moisture or fluid therethrough; otherwise the desired wall stability is not maintained. More particularly, if liquid gets past the coating and into the shale formation, the formation will "sluff", buckle or swell, and thus cause substantial well hole problems.
A wide variety of drilling fluids, usually clay-based muds or dispersions, have been extensively employed. These fluids have included various types of additives such as chrome lignosulfonates, lignins, tannins, oxidizing agents, etc. These additives may have a variety of functions or purposes, e.g. to serve as thinners or to improve fluid loss control. One such available additive is known as "Temflo-C". This product, which is used as a thinner, is a sulphomethylated reaction product comprising lignin, quebracho, sodium sulfite, paraformaldehyde and sodium hydroxide. The composition also includes sodium dichromate as oxidizing agent.
The principal object of the present invention is to provide an additive for gas and oil well drilling fluids, notably aqueous clay-based dispersions, which effectively functions to improve the fluid rheology and fluid loss control. A more specific object is to provide such an additive which can be used as a thinner for conventional drilling fluids and to improve the wall cake or coating obtained therewith so as to reduce fluid loss and to provide improved wall stabilization. Other objects will also be hereinafter apparent.
The invention is based on the finding that gilsonite may be advantageously reacted with lignite and a tannin, particularly quebracho, together with sodium sulfite, paraformaldehyde and sodium hydroxide, in water, to give a reaction product which is eminently suitable for use as a drilling fluid additive to improve rheology and fluid loss control.
The additive of the invention may be used with conventional gas or oil well drilling fluids. These are usually clay-based aqueous dispersions or muds. The present product may be used as the primary or sole additive in a manner similar to chrome lignosulfonate (CLS) additives or it may be used in combination with such other additives. Since the present additive does not contain any heavy metals, it has the advantage of being suitable for use in environmental areas where heavy metals cannot be used, to provide the desired rheological properties and fluid loss control. On the other hand, the product also demonstrates its indicated advantages when used with heavy metal-based additives. Thus, the present additive can be effectively used with CLS to provide a system with appreciably improved rheological properties and fluid loss control. It is also contemplated, as exemplified herein, that the additive will be used with conventional oxidizing agents, e.g. sodium dichromate, to obtain optimum results. Advantageously the present additive is dry blended with oxidizing agent to give a product which demonstrates highly advantageous rheological properties that are retained and even enhanced at extremely high thermal conditions. Preferably such a dry blend is pre-packaged to insure proper ratios of additive to oxidizing agent although, if desired, such mixture or other type of mixture can be prepared at the wall site. Modification of the additive to include an oxidizing agent such as sodium dichromate gives a more stable product, especially for use at particularly elevated temperatures. Without the oxidizing agent ten minute gel strengths (a standard test) may be excessive and the tolerance of the additive for elevated temperature and low gravity solids when used by itself may not be completely satisfactory in certain situations.
The use of sodium sulfite, paraformaldehyde and sodium hydroxide to sulphomethylate and causticize mixtures of lignins and tannins is known (see, for example, the Temflo-C product referred to earlier). The invention, however, also requires the presence of gilsonite in the reaction product, as noted. It appears that the reaction solubilizes the gilsonite component because the reaction product is essentially watersoluble.
The gilsonite component appears to be particularly important to control fluid loss and to act as a shale inhibitor. This component by itself would tend to cause an increase in fluid viscosity but this is effectively dealt with by the other components which make up the additive of the invention.
As indicated earlier, the additive of the invention comprises the product obtained by reacting together in the presence of water, quebracho, lignite, gilsonite, sodium sulfite, paraformaldehyde and sodium hydroxide. Each of the indicated components is essential to obtain optimum results according to the invention.
The reaction conditions used to make the product of the invention can be fairly widely varied. Usually, however, the product is prepared by heating the indicated components in a sealed reaction vessel at a temperature in the order of 100.degree. to 150.degree. C. for a period of 120 to 30 minutes, it being appreciated that shorter times can be used as the temperature is increased. It appears that the reaction results in sulphomethylation and caustization of a complex formed between the quebracho, lignite and gilsonite although the chemistry involved is highly complex and not fully understood. In any case, the resulting composition, which is essentially water-soluble, is preferably dried and may be stored for subsequent blending with conventional clay-based aqueous drilling dispersions or drilling fluids. This blending advantageously is carried out on site although it will be appreciated that the additive may be pre-packed with the drilling fluid.
The proportions of the reactants used to make up the additive of the invention may be relatively widely varied. Preferably, however, the reactant proportions fall within the following ranges on a solids basis:
______________________________________ % Solids ______________________________________ Quebracho 15-45% Lignite 25-45% Gilsonite 10-15% Sodium Sulfite 10-25% Paraformaldehyde 1-10% Sodium hydroxide 2-10% ______________________________________
While proportions outside these ranges may be used, it is noted that the gilsonite should not exceed 20% by weight, on a solids basis. The sodium hydroxide should also be used in amount sufficient to solubilize the gilsonite.
Advantageously the additive is prepared by mixing together the indicated reactants with water in a ratio of 40% solids, 60% liquid (water). However, it will be appreciated that this ratio can be widely varied without substantially effecting the desired results. The optimum ratios for any particular situation will depend on a variety of other factors, e.g. the water content of the reactants, water hardness, the sort of mixers which are available, drying time, the economics involved, etc. Usually, however, the ratio of water to reactants (solids basis) will fall in the range of 40-80% liquid and 80-40% reactants although, as noted, the ratio is not critical and will be selected in any particular situation to fit in with other operating conditions.
The reactants employed to make the additive are all commercially available and may be used in this form. The quebracho used in the examples which follow was the pure extract in block form which was pulverized into a powder before mixing with other reactants. It is also possible to use spray dried clarified pure quebracho extract. Alternatives to quebracho include wattle, divi-clivi or chestnut. The tannin content is the determining factor for the tannin material and amount thereof utilized. Preferably a minimum of 72% by weight of tannin is available in the material used.
The lignite used in the examples herein was a crushed commercial grade of mixed processed lignite. It is preferred that the lignite used be at least 75-85% soluble in a caustic (NaOH) solution.
Typically the gilsonite is that available as "American Gilsonite". This is a pulverized GP grade gilsonite with a flash point of 600.degree. F./316.degree. C. and a softening point of 320.degree. F./160.degree. C. This gilsonite was used in the ensuing examples although it will be appreciated that any available gilsonite may be used for present purposes.
The invention is illustrated by the following example wherein parts and percentages are by weight unless otherwise stated: